Project Summary The biological springs that act in parallel and in series with skeletal muscle can significantly influence the force, power and displacement of muscle fibers during locomotion. All movements involve dynamic interactions between passive elastic structures and muscle contractile elements, but our understanding of the consequences of this interaction for both normal and pathological gait is limited. Our long-term goal is to define the mechanical influence of elastic elements on muscle force production and gait. This project aims to understand how the mechanical interaction between internal muscle elastic elements and forces produced by muscle fibers define the gear ratio with which muscles operate. Previous work shows that this interaction significantly influences muscle force and speed, and we hypothesize that the influence of internal elastic elements on muscle shape changes underlies deficits in muscle performance in aging and many neuromuscular disorders. The project aims to construct and test a model of muscle shape change that can predict the influence of muscle force and muscle elastic properties on the gear ratio with which pennate muscles produce force. This project combines unique animal model systems with novel measurement techniques to make direct measurements of muscle force, muscle shape, and fiber length change both in vivo and in isolated muscles. The specific aims of the project are: 1) to develop a predictive model of muscle shape change and gearing 2) to use a series of studies of muscle function in vivo and in situ to test and refine this model; 3) to test the hypothesis that external constraints can influence muscle force output via their effect on muscle bulging, and 4) to test the hypothesis that restrictions to muscle bulging due to stiffened extracellular matrix reduce muscle force output in aged muscles. Changes in the elastic properties of connective tissue elements within muscle are associated with several neuromuscular disorders, aging, and occur secondarily to stroke and spinal cord injury. A fundamental understanding of the influence of these elements on muscle mechanical function will inform the design of rehabilitative strategies and interventions to improve muscle- tendon function. An improved understanding of how muscle elastic elements influence the mechanical behavior of healthy muscle-tendon units may also aid in the design of prosthetic devices.